The number of watts is found by multiplying the voltage by the amperage. In the case of the sounder and battery used as an example to explain Ohm’s Law, and where the voltage was 10 and the amperage found to be 2, the number of watts is 10 x 2, or 20 watts.
Seven hundred and forty-six watts represent one electrical horse-power. One thousand watts are called a kilo-watt.
The Coulomb
So far, none of the units have taken into consideration the element of time.
If water should be permitted to run out of a pipe into a tank until ten gallons had passed it would not be possible to tell at what rate the water was flowing by knowing that ten gallons had passed unless it were also known how long the water had been flowing. Ten gallons per minute or ten gallons per hour would indicate the rate of flow.
One ampere flowing for one second is the electrical unit of flow. This unit is called the coulomb.
One ampere flowing for one hour is called an ampere hour. The number of ampere hours is found by multiplying the current in amperes by the time in hours.
A battery may be said to have a capacity of 10 ampere hours. This means that it will deliver one ampere for 10 hours (1 ampere x 10 hours = 10 ampere hours) or 2 amperes for 5 hours (2 amperes x 5 hours = 10 ampere hours).
The same element of time enters into consideration in connection with the watt. One watt flowing for one hour is a watt hour and one kilowatt flowing for one hour is a kilo-watt hour.